1. Field of the Invention
The present invention relates to a control device for one kind of vehicle running simulator used in a motor vehicle plant and a vehicle repair shop, that is to say, a chassis dynamometer capable of simulating a running condition of a vehicle on an actual road while having a car body stand still by absorbing a force generated by the vehicle. More particularly, the present invention relates to a chassis dynamometer system adapted to form a control signal for the dynamometer on the basis of a control equation comprising desired parameters such as signals detected by a torque sensor and a speed sensor, the inertia of vehicle and a mechanical inertia of a system and control coefficients, in order to control a force to be absorbed by the dynamometer connected to a rotating roller, on which a driving wheel of the vehicle to be tested is placed in an interlocked manner through a shaft provided with a flywheel.
2. Description of the Prior Art
The general rough construction of a chassis dynamometer system is generally shown in a block diagram (FIG. 5).
Referring now to FIG. 5, reference numeral 1 designates a rotating roller on which a driving wheel of a vehicle to be tested is placed; numeral 2 designates a dynamometer used as a power-absorption device connected to said rotating roller 1 in an interlocked manner through a shaft 3; numerals 4, 5 and 6 respectively designate a flywheel, a torque sensor and a speed sensor mounted on the shaft 3; numeral 7 designates an electrical power convertor adapted to electrically absorb a force corresponding to a difference between a force generated by the vehicle and a force based on a mechanical inertia of the system by the rotating roller 1, the shaft 3, the flywheel 4 and by controlling a field current or an excitation current to the dynamometer 2 on the basis of a control signal output from a control device 8. In short, this chassis dynamometer system is adapted to absorb a force generated by the vehicle by combining a mechanical inertia-adjustment method with an electrical power-adjustment method, the mechanical inertia being set in a switched-over manner by switching-over a weight of the flywheel 4 according to the type of vehicle, and an electrical force of the dynamometer 2 being suitably adjusted by the control device 8. The control device 8 has included one type using a control method which operates on the basis of a speed-related equation and one type using a control method which operates on the basis of a torque-related equation, but recently it has been found that the latter control method which operates on the basis of the torque-related equation is advantageous in view of its control characteristics when the system is accelerated.
The control method on the basis of the torque-related equation includes one in combination of a feedforward control and a feedback control as already proposed in now abandoned U.S. application Ser. No. 624,962 to Kawarabayashi and one minimizing an evaluation function by a steepest descent method as disclosed in copending U.S. application Ser. No. 634,117 to Kawarabayashi, now U.S. Pat. No. 4,656,576.
FIG. 6 shows one example of a DC chassis dynamometer having the conventional construction in which the former control method in combination with a forwardback control and a feedback control is adopted.
That is to say, in many cases with this construction, a force F.sub.VEH (t) generated by the vehicle to be tested at a time t is measured by an output F.sub.TT (t) of the torque sensor 5 while a speed V(t) is measured by the speed sensor 6. In addition, these measured values F.sub.TT (t), V(t) are put in a feedforward control circuit 9 and an error-function operational circuit 10 in the control device 8.
In the feedforward control circuit 9, a force F.sub.PAU (t+.DELTA.t) to be absorbed by the dynamometer 2 in the subsequent step (at a time t+.DELTA.t) is given from said measured value F.sub.TT (t), V(t) and various kinds of parameter set at a time t by the following equation: ##EQU1## wherein I.sub.m : A set value of mechanical inertia (approximately represented by an inertia of the flywheel 4);
I.sub.r : An inertia of a roller in the dynamometer 2; PA1 I: An inertia of a vehicle to be tested; PA1 I.sub.e : An electrical inertia {=I-(I.sub.m +I.sub.r)} PA1 RL(V): Road load(=A+BV+CV.sup.X ; wherein A, B, C are constants); PA1 L.sub.m (V): A mechanical loss of the flywheel 4; PA1 V: A speed V(t) of a vehicle at a time t. PA1 I.sub.m : A set value of a mechanical inertia (approximately represented by an inertia of the flywheel 4); PA1 I: An inertia of the vehicle to be tested; PA1 RL(V): Road load (=A+BV CV.sup.x ; wherein A, B, C are constants); PA1 L.sub.m (V): A mechanical loss of the flywheel 4; PA1 V : V(t); PA1 F.sub.PAU '(t+.DELTA.t : The actual predicted output value of the dynamometer 2.
On the other hand, an error function is operated on in said error-function operating circuit 10. The error function is expressed by the following equations for determining an integrated value of a difference between an actual predicted output value F.sub.PAU '(t+.DELTA.t) of the dynamometer 2 including an inertia of the roller 1 and the desired value F.sub.TT +L.sub.m (V). ##EQU2##
An error signal from this error-function operating circuit 10 is put in an adder 12 through a feedback control circuit 11 which carries out a control action so as to make the error signal 0, where the signal F.sub.PAU (t+.DELTA.t) from the feedforward control circuit 9 is calibrated. In the case where the feedback control circuit 11 carries out, for example, a PI control, a calibrated signal is expressed by the following equation: ##EQU3## K.sub.p : A control coefficient of a proportional term of the PI control; K.sub.I : A control coefficient of an integration term of the PI control;
The above described calibrated signal F.sub.pc (t) controls the electrical power convertor 7 and outputs the field current or the excitation current through the dynamometer 2, whereby the dynamometer 2 is controlled so as to absorb a mechanical force output from the vehicle to be tested.
In addition, FIG. 7 shows one example of a DC chassis dynamometer having the conventional construction in which a control method is adopted so as to minimize the evaluation function by the latter steepest descent method.
That is to say, with this construction, a force F.sub.VEH (t) generated by the vehicle to be tested at a time t is measured as the output F.sub.TT (t) by the torque sensor 5 and fed to an evaluation function gradient operational circuit 13 in the control device 8 while the speed V(t) is measured by the speed sensor 6 and fed to said evaluation function gradient operational circuit 13 through an acceleration-operating differential circuit 14 in the control device 8.
In this evaluation function gradient operational circuit 13, a gradient .gradient.J of the evaluation function J={F.sub.TT (t)+L.sub.m (V)-F.sub.PAU '(t+.DELTA.t)}.sup.2, which is used as a function corresponding to the above described error function, is given on the basis of the following equation: EQU .gradient.J=F.sub.TT (t)+L.sub.m (V)-RL(V)-(I-I.sub.m)dv/dt
wherein
The output signal (the gradient .gradient.J) from the evaluation function gradient operational circuit 13 is fed to a control coefficient multiplier circuit 15 and .alpha..multidot..gradient.J, where .alpha. is a small control coefficient, is output from the control coefficient multiplier circuit 15 and fed to a successive calibration operational circuit 16.
Thereupon, the successive calibration operational circuit 16 outputs a control signal F.sub.PAU (t+.DELTA.t) in the next step by the following equation on the basis of the steepest descent method from the control signal F.sub.PAU (t) in the preceding step accumulated in a memory 16A and the .alpha..multidot..gradient.J. ##EQU4##
In the successive calibration operational circuit 16, comprising the memory 16A and a feedback subtracter 16B, a control signal P.sub.PAU (t+.DELTA.t) of the electrical power convertor 7 is subjected to a successive calibrational operation by the steepest descent method as described above so as to meet changes of the system such as an acceleration and a deceleration, whereby a field current or an excitation current supplied to the dynamometer 2 is controlled and the dynamometer 2 generates a force absorbing a mechanical force output by the vehicle to be tested within an appointed time.
However, in an operation control device of a chassis dynamometer having the above described conventional construction, whether using a control method in combination with a feedforward control and a feedback control as shown in said FIG. 6 or using a control method minimizing an evaluation function using the steepest descent method as shown in said FIG. 7, use control coefficients (K.sub.p and K.sub.I in the control method in combination with a feedforward control and a feedback control and .alpha. in the control method minimizing an evaluation function by the steepest descent method) in the control equation comprising desired parameters, such as a torque, a speed, an inertia of a vehicle and a mechanical inertia of a system, and these control coefficients are set at certain appointed values, so that disadvantages such as instability, significant delay in the settling time of the control system and oscillation have occurred according to a change of a measuring condition due to a difference in the kind of a vehicle and the like. In the case of such an instability, it is necessary to adjust the control coefficients. Such an adjustment must be carried out by a trial and error method, and it has been very difficult to carry out an exact adjustment.